The present process is an improvement over the processes described in U.S. Pat. Nos. 3,919,393 and 3,919,394, which are assigned to the assignee of the present invention. In those processes, an improvement in sulfur dioxide removal from flue gases was provided, where the addition of the specified amount of magnesium components to a lime slurry scrubbing system resulted in increased removal of the sulfur oxides. In those processes, a calcium oxide aqueous slurry containing a specified amount of magnesium components was used as the scrubbing medium in a wet scrubbing unit. While those improved processes work well and are commercially utilized, problems can be associated with the use of aqueous slurries in wet scrubbing units. In such slurries, suspended solids are present which have a tendency to accumulate not only in the holding tanks for the slurry but also in pipelines and nozzles, which can result in plugging. In addition, those processes tend to produce a sludge for disposal that has relatively poor dewatering characteristics, and sufficient dewatering capacity has to be provided in the design of a system using those processes to accommodate the poor dewatering characteristics of the sludge produced.
In U.S. Pat. No. 4,014,978, a method for removing sulfur dioxide from flue gases is described using a solution containing excess calcium or magnesium bicarbonate. Reaction of the sulfur dioxide with the bicarbonates produces soluble bisulfites, with oxidation of the resultant solution effected. The oxidized solution is then contacted with an alkaline earth metal carbonate to regenerate a bicarbonate that is recycled to the scrubbing unit. Precipitated sulfate and other solids are separated from the regenerated bicarbonate solution.
U.S Pat. No. 4,193,971, Kawamata et al. contact an aqueous slurry containing magnesium hydroxide and gypsum with an SO.sub.2 -containing gas to fix the SO.sub.2 as magnesium sulfite, contact the resulting slurry with an oxidizing agent to convert the magnesium sulfite into magnesium sulfate, subject the aqueous slurry obtained to filtration, treat the magnesium sulfate with calcium hydroxide in the presence of gypsum crystals, as seed crystals, to obtain magnesium hydroxide and gypsum, and then recycle the latter slurry to the initial step.
The oxidation of effluent from a lime scrubbing process to convert sulfites and bisulfites for the purpose of easy separation from the aqueous medium as precipitated sulfates is a known procedure. In U.S. Pat. No. 4,046,856, for example, a slurry of a calcium compound, such as calcium hydroxide or calcium carbonate, has a soluble magnesium compound added thereto, for reaction with sulfur dioxide in a scrubbing unit, with the effluent from the scrubbing unit oxidized to form sulfates. Sulfuric acid is also added to the oxidizing medium for pH control. The discharge from the oxidizer is passed to a thickener in which gypsum is separated, and the magnesium compounds returned to the tanks wherein a calcium slurry is formed for passage through the scrubbing unit, the slurry composed mainly of calcium hydroxide, calcium sulfate and magnesium hydroxide in water.
Abrams et al. in U.S. Pat. No. 4,246,245 contact a SO.sub.2 -containing gas with a hydrated lime to remove SO.sub.2 therefrom. In one embodiment, a type S hydrated dolomitic lime is used as a source of magnesium sulfite, resulting in the formation of magnesium sulfite-bisulfite. A sidestream of the latter is oxidized to magnesium sulfate and the magnesium sulfate is converted to magnesium hydroxide, which is combined with the wet scrubber effluent to provided magnesium sulfite.